1. Field of the Invention
The present invention relates to a method for producing a hydrogen storage alloy which can electrochemically absorb and desorb hydrogen in a reversible manner.
2. Description of the Related Art
Storage batteries, which are widely used as power sources in a variety of applications, are typically classified into two general groups of lead-acid storage batteries and alkaline storage batteries. Between the two groups, alkaline storage batteries tend to be more reliable, and can be made smaller and lighter. Small alkaline storage batteries are generally favored for portable electric appliances, while large alkaline storage batteries have been used mainly in conjunction with industrial equipments.
While some alkaline storage batteries use air or silver oxide for their positive electrode material, majority of the positive electrodes are of nickel. Nickel electrodes have been particularly popular since they were reconfigured from a pocket type to a sintered type, and became even more popular with the development of hermetic-sealing.
Cadmium is most commonly used to form the negative electrode of alkaline storage batteries, however other materials, including zinc, iron, hydrogen, and the like have also been employed.
There is considerable commercial interest in storage batteries that have a higher energy density than batteries currently available. Toward achieving this goal, researchers have investigated nickel-hydrogen storage batteries which comprise hydrogen storage electrodes. A number of proposals have been made on the production method of the hydrogen storage electrodes using metal hydrides.
The alloys in these electrodes, or the hydrides form of such alloys, can absorb and desorb hydrogen in a reversible manner, and thus the alloys and the electrodes made from these alloys have come to be known as hydrogen storage alloys and hydrogen storage electrodes (or hydrogen storage alloy electrodes), respectively.
Batteries made with hydrogen storage electrodes have a larger theoretical energy density in comparison with batteries formed with cadmium electrodes. Also, batteries employing hydrogen storage electrodes are not susceptible to the formation and subsequent deformation of dendrites, which is a problem with zinc electrodes. These advantageous properties, as well as the promise for a longer cycle life and a reduction in the environmental concerns inherent in zinc- or cadmium- containing electrodes/batteries, have encouraged research into developing alloys suited for hydrogen storage electrodes, particularly negative electrodes for alkaline storage batteries.
Prior art alloys for hydrogen storage electrodes include multi-component alloys such as those of either Ti-Ni system, or La- (or Mm-) Ni system (where Mm is a misch metal). The multi-component alloy of the Ti-Ni system is classified as an AB type (where A is La, Zr, Ti or an element with a similar affinity for hydrogen, and B is Ni, Mn, Cr or any other transition metal). When this type of alloy is used as the negative electrode in an alkaline storage battery, the electrode exhibits a relatively large discharge capacity during the initial charging and discharging cycles. However, electrodes comprising these alloys have a disadvantage that they can not maintain their large discharge capacity after repeated charging and discharging cycles, i.e., do not have large saturation discharge capacities.
Another multi-component alloy is of the La- (or Mm-) Ni system, which is classified as an AB.sub.5 type, where A and B are defined as above in relation to the AB type of alloy. A number of research project have recently been developed for the alloys of this system and thus the alloy have been regarded as a relatively promising alloy material for the electrodes, so far. However, the alloys of this system have several disadvantages such that they have a relatively small discharge capacity, that they have insufficient cycle life performances as the electrodes of the batteries, and that their material cost is expensive. Therefore, there has been a demand for novel alloys usable for making hydrogen storage electrodes having a large discharge capacity and a long cycle life.
A Laves phase alloy of an AB.sub.2 -type (where A is an element with a large affinity for hydrogen such as Zr or Ti, and B is a transition metal such as Ni, Mn or Cr) has the potential to overcome many of the shortcomings of the multi-component alloys described above. Electrodes for a storage battery formed from a Laves phase alloy of the AB.sub.2 -type have relatively high hydrogen storing capability and exhibit a large discharge capacity and a long cycle life. The alloys of this system however had a disadvantage that their material cost was expensive in comparison with the hydrogen storage alloys of the Ti-Ni system and the Mm-Ni system, because they contained expensive metal components such as Zr or V in large quantities.
In order to overcome the above-discussed disadvantages of the Laves phase alloy of an AB.sub.2 -type, there has been proposed a reduction diffusion method. According to the reduction diffusion method, in place of expensive metal materials, an inexpensive raw material containing their oxides is used, and metal calcium in an amount sufficient for reducing the oxides is mixed with the oxides, and then the mixture is subjected to a heat treatment for the reduction.
The prior art reduction diffusion method employing metal calcium as its reducing agent is a process for producing an alloy by allowing the reduced metals, for instance Zr, Ti, V, Mm and the like, as well as other metals, for instance Mn, Cr, Co and the like which have initially been incorporated in the raw material, to diffuse into the nickel powder which have been incorporated in the raw material.
It is however difficult to obtain a hydrogen storage alloy having a sufficient hydrogen storing capability by the prior art reduction diffusion method for the following reason. Depending on the particle size of the raw material nickel and the temperature distribution in the heating furnace, the reducing reaction in the prior art method would not sufficiently proceed in the interior or core of the nickel particles. Some amount of nickel would remain unreacted in the center of the particles and the alloy composition would deviate from its aimed composition depending on the regions in the produced alloy mass. This deviation in the alloy composition would deteriorate the homogeneity of the produced alloy.